The pathology and biology of selected experimentally induced and naturally occurring neoplasms of rodents were characterized in order to better elucidate their pathogenesis. Leu-7, a cell surface glycoprotein on human natural killer cells, was found to be an oncofetal antigen of hamster olfactory epithelium, when hamsters were injected with N-nitrosodimethylamine. This antigen, however, was not found in fetuses of other rodents or in their olfactory tumors. So-called "Clara cell" papillary tumors of mouse lung were found to arise from pulmonary alveolar walls and not from Clara cells. Retroviral antigens, including HTLV-III p24 were found in fixed tissue sections from autopsy specimens of humans, monkeys and mice by ABC immunocytochemistry utilizing both monoclonal and ppolyclonal antibodies. Cross-reactions between STVL-III and HTVL-III were seen. In monkeys, cerebral glia and macrophages contained abundant viral antigens. Retroviral antigens were abundant in early stages of simian and human AIDS but were considerably less prominent in the later stages of these diseases.